Regex any ASCII character

If you really mean any and ASCII (not e.g. all Unicode characters):

xxx[\x00-\x7F]+xxx

JavaScript example:

var re = /xxx[\x00-\x7F]+xxx/;

re.test('xxxabcxxx')
// true

re.test('xxx☃☃☃xxx')
// false

You can use the [[:ascii:]] class.

Since US-ASCII characters are in the byte range of 0x00–0x7F (0–127):

xxx[\x00-\x7F]+xxx

[ -~]

It was seen here. It matches all ASCII characters from the space to the tilde.

So your implementation would be:

xxx[ -~]+xxx